Method for introducing a borehole

ABSTRACT

Methods for constructing a borehole, in some examples, in the Earth&#39;s crust, may involve holding a drill head in the borehole by a linkage. The drill head may include a thermal device that causes material on a base of the borehole to be released from the solid phase via phase change. The released material may be removed in a direction of the Earth&#39;s surface. Further, the thermal device may be operated so that it generates a thermal output power high enough to predominantly sublimate material when transitioning out of the solid phase.

The invention relates to a method for introducing a borehole, particularly into the Earth's crust.

Mechanical drilling methods are conventionally used today in order to exploit oil and natural gas sources. Said mechanical drilling methods apply a rotating bit in order to remove the rock from the borehole. In addition to the rotating method, there are also percussive or rotary-percussive drill bits. The drill bit is driven mechanically or hydrodynamically. Linkages, which are commonly screwed together or inserted in sections, transmit mechanical energy to the bit to remove rock material. Cooling is required for this process. Cooling is performed by a drilling fluid that consists, to a great extent, of water. In addition to cooling, the fluid is also used to transport the removed drill cuttings from the base of the borehole upward. However, said cooling and removal methods are limited by the high temperatures which prevail at depths, particularly at greater than 2,000 m. The temperatures here are high enough that the drilling fluid can no longer perform effective cooling. This is one of the reasons why drilling at depths of greater than 2,000 m is difficult to perform. Above a certain temperature, the cooling fluid begins to boil and thus can no longer discharge sufficient heat or rock. The depths that can be reached are also limited by the respective geological conditions of the rock in the respective borehole. The boiling point of the drilling fluid can be increased by various additives, which enables its functionality even at high temperatures; however, there are technical limits to these adjustment possibilities.

WO 2013/135391 A2 discloses a method for introducing cavities into rock, wherein the rock on the front of the cavity is thermally melted. The liquefied rock is removed from the cavity using a gaseous medium. The heat required in order to melt the rock is provided by a plasma generator arranged on a tunneling head. The high temperatures in the borehole create no significant disadvantages for this method.

Handling of the liquefied rock is problematic with plasma drilling, however, as it must be conveyed past the drill head to the opening of the borehole. The liquefied rock can precipitate (condense) on the drill head. This can lead to destruction of the drill head, which creates high costs and downtime. Until now, this problem has been approached by keeping the fluid level at the base of the borehole as low as possible. The power of the plasma generator is reduced for this purpose. This naturally slows the drilling progress, since the feed rate is extensively linear in relation to the thermal output power. In this respect, plasma drilling is currently rarely applied cost-effectively.

The invention aims to solve the problem of providing an improved method for constructing boreholes, which is particularly characterized by fast advancing and by high endurance.

Said problem is solved by a method for constructing boreholes, particularly into the Earth's crust, by means of a drill head which is held in the borehole on linkages, wherein the drill head comprises a thermal device which causes material, particularly rock, on the base of the borehole to be released from the solid phase via phase change, wherein the released material is taken away in the direction of the opening of the borehole, particularly to the Earth's surface. According to the invention, the thermal device is operated so that it generates a high thermal output power, by means of which the material predominantly sublimates when transitioning out of the solid phase.

The core of the invention lies particularly in the fact that, due to sublimation in a thermal drilling method, the material does not transition into the liquid phase at all. In fact, said phase is skipped over by sublimation. The risk of precipitation of liquid material on the drill head is immensely reduced as a result. The risk of liquid rock splashing onto the drill head and precipitating there is also reduced. In contrast to the plasma drilling according to the prior art described above, the power input is consequently not reduced according to the invention, but rather instead increased in order to prevent the formation of liquid material. Increasing the thermal power on the material reduces the melting depth to less than 1 cm, which leads to a significant reduction in the proportion of liquid material on the base of the borehole; this is due to the short-term cooling effect of the sublimation on the subjacent material layers. By increasing the thermal output power, the possible feed rate simultaneously increases.

Furthermore, sublimation of the material enables fast removal of the material. Immediately after the torch or, in individual cases, controlled by cooling nozzles, the material resublimates into small particles which can be easily flushed out. Unlike methods that transition through a liquid phase, the particles created during resublimation are significantly smaller than the particles created by condensation.

So-called plasma torches are particularly used as the thermal device, whereby the expression “torch” is sometimes used incorrectly in this context. The present method depends on the high temperatures that the device generates; however, this does not necessarily have to be accompanied by burning, or oxidation. Optical devices, such as lasers, are also generally conceivable, if they can provide the required thermal power.

At least 50 wgt.-%, preferably at least 80 wgt.-%, at least 90 wgt.-% or at least 95 wgt.-% of the material released from the solid phase via sublimation transitions into the gaseous phase. The rest of the released material first melts and only then transitions to the gaseous phase, if at all. The high proportion of sublimated material also causes an abrupt volume enlargement which removes any liquid components from the solid surface on the base of the borehole. In this respect, it is not necessarily required that the material be released from the solid phase only by sublimation.

In conventional surface treatment of rock pieces by means of a plasma jet, rock is also sporadically sublimated, as described in DE 19 43 058 C3, for example; a thermal drilling tool being intentionally brought to a power level that sublimates instead of melting the majority of the rock in order to solve the stated problem has not previously been described, however.

Feed rates of 2-10 mm/s can be achieved according to the invention. Under optimal operating conditions, plasma drilling also has potential for longer service life compared to mechanical drilling methods.

The phase state of the material to be released, particularly at the base of the borehole, is preferably monitored by at least one sensor attached on the drill head. The proportion of liquid material in the total output can thus be determined and measured initiated as required. To this end, the phase state of the drilling material on the base of the borehole is optically monitored by means of the sensor attached on the drill head. The proportion of the liquid phase in the total output can thus be continually determined. The sensor is particularly based on pyrometric temperature measurement and serves to determine the temperature difference between the released material on the base of the borehole and the side wall of the borehole. The method according to the invention utilizes temperature differences between the solid and liquid phase. The proportion of the liquid phase can be determined from the specification of the meniscus, using a mathematical method in connection with the flash pressure of the liquid phase.

A quantity of liquefied material on the base of the borehole is preferably regulated to a specified nominal value by regulating the thermal output power, wherein the thermal output power is increased for a reduction in the quantity of liquefied material. Such a regulation can ensure that the liquid proportion of released material does not become too great. By reducing the liquid proportion, the risk of clogging the borehole is kept low, without also reducing the feed rate.

There are special conditions in deep boreholes of more than 1000 m in depth. The rock there particularly has one or more of the following parameters:

Density: 1300-4000 kg/m3;

-   -   Thermal conductivity: 2-5 W/m K;     -   Specific thermal capacity: 600-2000J/kg K;     -   Melting point: 600-2000° C.;     -   Boiling temperature: 2800-4000 K     -   Evaporation enthalpy: 2 MJ/kg;

The borehole particularly exhibits the following parameters:

Distance from surface of the Earth to the base of the borehole (depth of borehole): at least 1,000 m, particularly at least 2,000 m or at least 4,000 m.

Diameter of the borehole 2-30 cm, particularly less than 20 cm.

The method described here is particularly suitable for producing boreholes with a high aspect ratio (ratio of the depth to the diameter of the borehole) of at least 1,000:1 particularly of at least 3,000:1 or at least 10,000:1, or, in the case of very deep boreholes, at least 20,000:1 or at least 100,000:1.

The power of the thermal device—that is, the thermal output power occurring in the method—is at least 80 kW, preferably at least 1,000 kW.

If a plasma generator device is chosen as the thermal device, the temperature of the emitted plasma beam on the drill head should equal 2,000 K, preferably at least 5,000 K, in order to cause sublimation to the required extent. The following gases can be used: nitrogen, acetone, oxygen, hydrogen, helium, argon and carbon dioxide. The power density equals preferably at least 10⁷ W/m², preferably 5×10⁷ W/m². Power density is considered to be the thermal power per unit of area which is applied by the thermal device to the surface of the rock.

A gas stream is preferably used for this in order to convey the removed material toward the surface, particularly the surface of the Earth. This can be the same gas that is also used for a plasma jet. The material is then guided past the side of the drill head, particularly through a gap between the drill head and the borehole.

The sublimated material is preferably cooled by a cooling gas stream separate from the plasma jet. This preferably forms a gas cushion between the sublimated rock and the drill head. In particular, said cooling gas stream or gas cushion first of all ensures that the sublimated material does not come into contact with the drill head. Second of all, a cooling of the sublimated material can be effected so that resublimation and, as a result, a sort of dust collection or formation of the smallest of particles occurs. Said dust material is then conveyed upward through the gap. The resublimation can also occur directly on the wall of the borehole, so that the material deposits there and effects a vitrification of the borehole.

The cooling gas stream is preferably blown laterally into the gap between the drill head and the borehole. The gaseous material is thus prevented from coming into contact with the drill head and condensing and solidifying or resublimating thereupon.

The invention furthermore relates to a device for constructing a borehole, particularly into the Earth's crust. The device comprises a drill head, a linkage for holding the drill head in the borehole, and a thermal device arranged on the drill head, which causes the material on the base of the borehole to be released from the solid phase via phase change. According to the invention, the device furthermore comprises a sensor, particularly attached to the drill head, by means of which the phase state of the loosened material can be monitored, particularly on the base of the borehole. A photooptical sensor, particularly a pyrometer, can be used as a sensor. The regulating of the thermal output power described above can be implemented by means of such a device.

The invention is described below in greater detail based on the figures. Here,

FIG. 1 shows a borehole having a drill head introduced therein, in cross-section;

FIG. 2 shows a schematic of the borehole according to FIG. 1, having different characteristics of the liquid level on the base of the borehole.

FIG. 1 shows a borehole 1 which is introduced into the Earth's crust 3 from the surface of the Earth 7. The depth T of the borehole (=distance from the Earth's surface 7 to the base 2 of the borehole 1) equals approximately 4,000 m. The borehole is to be enlarged so that further depths can be penetrated. A drill head 4 is provided for said purpose which is held by a linkage 5, which extends coaxially to the borehole 7 from the Earth's surface 7 into the borehole 7. A plasma generator device 6, which generates a plasma jet 8, is arranged inside the drill head 4. By means of the plasma jet 8, which has a temperature of 2,000 K or more, rock 3 on the base 2 of the borehole 1 is released from the solid phase and thus cleared away.

The basic structure of the plasma generator device corresponds to already known devices of this type and comprises a central, internal anode 10 and an annular cathode 9, arranged coaxially to the anode 10. A gas suitable for forming plasma, such as nitrogen, oxygen, hydrogen, argon, helium or carbon dioxide, is blown at high pressure via a supply line 1 into the region between the cathode 9 and the anode 10. With correspondingly applied high voltage, the arrangement of the anode 10 and cathode 9 generates an electrical arc, by means of which the plasma or the plasma jet 8 is produced. As a result, the gas undergoes a temperature increase to more than 2,500 K, which is necessary for removal of the rock.

Therefore, the plasma jet 8 is brought to a power level that predominantly sublimates the rock and does not melt it first. Liquid rock is thus extensively prevented from collecting on the base 2 of the borehole 1. Liquid rock is to be avoided, since it easily sets on the drill head and can damage the drill head as a result. Furthermore, it can collect in the annular gap between the drill head and borehole, causing an obstruction there.

It must be ensured that the released gaseous rock returns to the solid phase as quickly as possible, and sublimates and solidifies as finely grained as possible. A shell channel 12 is formed for this purpose within the drill head 4, which is arranged in an annular manner around the plasma generator device 6. A cooling gas stream 15 flows through said shell channel 12, likewise originating from the supply line 11, at high speed. Said gas exits from the shell channel 12 near a face 17—that is, the downward pointing region of the drill head—and ensures that a sort of gas cushion 16 is formed between the plasma gas 13, together with the sublimated rock, and the drill head 4. Said gas cushion 16 is required where the rock is present in gaseous form, which is marked by the line drawn and identified with reference sign 13. Moreover, due to said gas cushion 16, prompt cooling of the gaseous rock occurs, causing it to resublimate and to thus take a solid, dust-like form. This is shown in the figure by the dotted line identified by reference sign 14. Mixing with the cooling gas stream 15 and common discharge of cooling gas stream 15 and plasma gas stream 14 together with resublimated rock then occurs in the direction of the Earth's surface 7.

The determination of the liquid level on the base of the borehole 2 is explained based on FIG. 2. A pyrometer 17 measures the temperature distribution on the borehole 1 in the region of the drill head 4. Solid components, such as the edge of the borehole 1, have a lower temperature than liquid components, namely the liquefied rock 18; liquid components have a lower temperature than gaseous components. The shape of the meniscus, or the curvature of the liquid surface on the base 2 of the borehole 1, can be determined on this basis.

The shape of the meniscus is linked to the liquid level on the base of the borehole. FIG. 2a shows a meniscus having a steep outer region, which indicates a low liquid level. FIG. 2b shows a meniscus having a flat outer region, which indicates a higher liquid level. A correlation between the shape of the meniscus and the liquid level is created via mathematical models.

REFERENCE SIGN LIST

-   1 Borehole -   2 Base of the borehole -   3 Rock/Earth's crust -   4 Drill head -   5 Linkage -   6 Plasma generator device -   7 Earth's surface -   8 Plasma jet -   9 Cathode -   10 Anode -   11 Supply line -   12 Shell channel -   13 Plasma gas stream with sublimated rock -   14 Plasma gas stream with resublimated rock -   15 Cooling gas stream -   16 Gas cushion -   17 Pyrometer -   18 Liquid layer T Bore hole depth 

1-11. (canceled)
 12. A method for introducing a borehole into Earth's crust, the method comprising: holding a drill head in the borehole by a linkage; operating a thermal device of the drill head so that the thermal device generates a thermal output power high enough to cause material on a base of the borehole to be released from a solid phase via phase change predominantly by way of sublimation; and removing the released material in a direction of an opening of the borehole.
 13. The method of claim 12 wherein at least 40% by weight of the material released from the solid phase via sublimation transitions into a gaseous phase.
 14. The method of claim 12 wherein at least 90% by weight of the material released from the solid phase via sublimation transitions into a gaseous phase.
 15. The method of claim 12 wherein at least 95% by weight of the material released from the solid phase via sublimation transitions into a gaseous phase.
 16. The method of claim 12 further comprising monitoring a phase state of the released material at the base of the borehole with a sensor attached to the drill head.
 17. The method of claim 12 further comprising regulating a quantity of liquefied material on the base of the borehole to a specified nominal value by controlling the thermal output power, wherein the thermal output power is increased for a reduction in the quantity of liquefied material.
 18. The method of claim 12 comprising operating the thermal device at a heating power of at least 80 kW.
 19. The method of claim 12 comprising operating the thermal device at a heating power of at least 1,000 kW.
 20. The method of claim 12 comprising operating the thermal device so as to generate a temperature of at least 2,000 K.
 21. The method of claim 12 comprising operating the thermal device so as to generate a temperature of at least 5,000 K.
 22. The method of claim 12 further comprising cooling at least the released material that has been sublimed by a cooling gas stream separate from a plasma jet.
 23. The method of claim 22 wherein the cooling gas stream forms a gas cushion between the released material that has been sublimed and the drill head.
 24. The method of claim 23 further comprising blowing the cooling gas stream into a gap between the drill head and the borehole.
 25. A device for introducing a borehole into Earth's crust, the device comprising: a drill head; a linkage for holding the drill head in the borehole; a thermal device disposed on the drill head that causes material on a base of the borehole to be released from a solid phase via phase change; and a sensor for monitoring a phase state of the released material at the base of the borehole.
 26. The device of claim 25 wherein the sensor is attached to the drill head.
 27. The device of claim 25 wherein the sensor is a photo-optical sensor.
 28. The device of claim 25 wherein the sensor is a pyrometer.
 29. The device of claim 25 wherein the thermal device causes the material on the base of the borehole to be released from the solid phase via phase change predominantly by way of sublimation. 